There exist rate-adaptive cardiac pacemakers that provide a stimulation rhythm (i.e., a pacing rate) delivered as a function of one or more control parameters linked to the physical activity of the wearer. These control parameters can be, for example, minute ventilation, blood temperature, oxygen concentration of the blood, pH of the blood, vibrations sensed at the pacemaker case, e.g., at the trunk of the wearer, low frequency acceleration at the trunk of the wearer, the QT interval, as well as other measurable parameters indicative of the activity level of the wearer.
Among all of the parameters measured by adequate sensors, the so-called parameter or parameters of enslavement (herein referred to as a "control" parameter), some appear to have a delayed reaction, as compared to that of the physiological cardiac rhythm, in response to a change in activity, and some change more rapidly as compared to the physiological cardiac rhythm. For example, in response to a change in activity, the temperature of the blood will change as an indicator of activity, but more slowly than the natural physiological cardiac rhythm will change in response to that activity. Hence, at an increased stable activity level, the physiological (natural) cardiac rhythm will stabilize before the blood temperature stabilizes. In contrast, sensed accelerations of the trunk will change and stabilize faster than the corresponding physiological cardiac rhythm to a change in the activity level. The relationship between these control parameters and the corresponding cardiac stimulation frequency to be reached, and the monitoring of those control parameters in various forms, are described in the art.
The U.S. Pat. No. 5,074,302 refers to the utilization of a control parameter that has a linear or non-linear relationship function. The document WO 92/03183 refers to relationships associated to one or several parameters. In these descriptions the change over time of the stimulation frequency is not specified in the case where the variation of the control parameter is more rapid than that of the physiological cardiac rhythm. As the inventors have realized, it is necessary to define how the cardiac stimulation frequency is going to change over time from its current value (i.e., a start frequency) to the value calculated based on the measured control parameter(s) (i.e., a calculated target frequency) and a given relationship.